This invention relates to improvements in an elevator speed control system for controlling the rotational speed of an induction motor in response to a command speed signal.
With the advance of the semiconductor technology, there have been recently utilized control systems for controlling the rotational speed of an induction motor by changing the voltage applied to the motor by a thyristor or thyristors. When applied, for example, to elevator systems, the conventional type of those control systems can have an overall transfer characteristic selected to be most suitable only for a given voltage applied to the induction motor. Under these circumstances, any voltage exceeding the given voltage causes an extraordinarily rapid response of the system and therefore unstable operation resulting in the movement of an associated elevator car becoming very vibrational. On the contrary, any voltage below the given voltage causes the systems to be extraordinarily slow in response. As a result, the elevator car tends to overshoot its position where it is to land. Alternatively it has been likely to be affected by various external disturbances.
Accordingly it is an object of the present invention to eliminate the disadvantages of the prior art practice as above described.
It is another object of the present invention to provide a new and improved elevator speed control system for stably controlling the rotational speed of an inductor motor for an elevator car with a high accuracy by always maintaining the overall gain thereof constant even though the induction motor produces a torque which changes in response to a variation in the voltage applied thereto.